Cryogenic refrigeration systems for High Temperature Superconducting (HTS) devices are well known. In one basic form, these systems comprise a cooling loop, a refrigeration unit and a coolant. The cooling loop, e.g., a configuration of pipe or other conduit, is arranged about a device that requires cooling, e.g., an HTS cable, and the loop is in fluid communication with the refrigeration unit. The refrigeration unit is a mechanical refrigeration device that is well known in the industry. Coolant, e.g., liquid nitrogen, flows from the refrigeration unit into the cooling loop, circulates through the cooling loop extracting heat from the device, and then returns to the refrigeration unit for removal of the heat and circulates back to the cooling loop.
Cryogenic refrigeration systems may be equipped with a backup or reserve refrigeration unit in the event the primary unit fails. Providing such complete redundancy in the event of the failure or routine maintenance of the refrigeration unit is generally not cost effective and adds complexity and physical size to the system.
Cryogenic refrigeration systems comprising two or more cooling loops, such as those used in connection with an HTS cable, would typically require one backup refrigeration unit per cooling loop. While effective, having one backup unit for each cooling loop adds to the capital expense of the overall refrigeration system and to its complexity of operation.
HTS power or transmission cables are also well known. These cables require cryogenic cooling, and representative HTS power or transmission cables are described in U.S. Pat. Nos. 3,946,141, 3,950,606, 4,020,274, 4,020,275, 4,176,238 and more recently, U.S. Pat. Nos. 5,858,386, 6,342,673 and 6,512,311. The configuration of a typical HTS cable is an HTS conductor or conductors cooled by liquid nitrogen flowing through either the hollow conductor core or in a fluid passage around the outside of the conductor(s). The attractiveness of HTS cables over conventional cables of the same size is that the former can carry multiple times the power than the latter, with almost no loss of electrical capacity.
The normal mode of cooling an HTS cable is to provide a mechanical refrigeration unit, known in the industry, to cool a closed loop of purely subcooled liquid nitrogen. “Subcooled” liquid nitrogen is nitrogen cooled to a temperature below its boiling point, which depends on the operating pressure. For example, at a closed loop operating pressure of 5 bar,abs the boiling point of liquid nitrogen is 94K. At a typical coolant temperature of from 70-75K, the liquid nitrogen would be subcooled in an amount of 19 to 24 degrees. Typically, a single subcooled liquid loop cannot cool the entire length of the cable and, accordingly, there must be multiple manageable segments. In present arrangements, backup refrigeration capability is provided, if at all, on an individual segment basis. Illustrative is the HTS cable and cooling system described in EP 1,355,114 A2.
The HTS cable and cryogenic cooling system of EP '114 comprises first and second cooling channels (4,5) about an HTS cable. Liquid nitrogen is circulated through these channels in which it picks up heat from the cables, passes to a low pressure, boiling liquid nitrogen bath (9), i.e., a subcooler, in which the heat is removed from it, and then it is circulated back to the channels. If liquid nitrogen is lost from the system for any reason, makeup nitrogen is added to the system from a storage tank (1). The storage tank and its connecting hardware is designed to provide initial nitrogen required to charge, and replenish as necessary, the cooling system. The storage tank also provides the coolant required for initial cable cool down through a liquid and gaseous nitrogen mixing system.